Except for using a built-in auxiliary power unit to start a jet engine, another alternative is by using an air start unit. While using an air start unit for starting a jet engine, compressed air provided by the air start unit is being fed into the air turbine of the jet engine for driving the air turbine to rotate that brings along the main shaft of the jet engine coupling to the same also to rotate. As the rotation speed of the main shaft reaches a specified threshold, the jet engine can be ignited.
The air start unit is substantially a trailer-mounted power unit capable of providing AC and DC electrical power as well as high volume air for starting aircraft engines. The unit consists of a gas turbine engine, air compressor, AC generator, DC generator or DC rectifier, fuel system, battery, air start hose with coupling, and electrical systems for starting and control functions, whose structure and functions are similar to those of the auxiliary power unit (APU). The amount of airflow that an air start unit can provide is closely related to the gas turbine engine and air compressor arranged therein.
As seen in FIG. 1, which is a cross-sectional view of a rotor assembly according to a conventional compressor, the rotor assembly 10 comprises a main body 11, two bearings 12, 13, and a transmission shaft 14; wherein the main body 11 has a hole (not shown in the figure) at the center thereof and an impeller 15 disposed surrounding the hole, where the bearings 12, 13 are being arranged in the hole and are disposed respectively at a side of the impeller 15 for enabling the transmission shaft 110 to pass through the bearings 12, 13 and further connect to a power output mechanism (not shown in the figure).
In addition, the rotor assembly 10 further comprises an inlet track 16 and a windpipe 17, both being circumferentially disposed about the hole, wherein the impeller 15 is disposed in the inlet track 16 and the volume thereof occupies the second half of the inlet track 16. Yet, the bearings 12 is substantially a ball bearing and the bearing 13 is substantially a roller bearing capable of rolling axially, and a mechanical oil seal 18 is disposed at the periphery of the two bearings 12, 13 to prevent the lubricating oil from spilling out. Therefore, when the transmission shaft 14 is driven to rotate by coupling the same to a shaft of a power output mechanism, the impeller 15 is rotating that the air sucked in from the inlet track 16 is pressure-boosted by the impeller 15, and thereafter, the compressed air is discharged from the windpipe 17.
However, the rotor assembly of the conventional compressor is still suffer from the following drawbacks:    1. The impeller is disposed between the ball bearing and the roller bearing. Therefore, when the impeller is rotating, the lubricating oil of the bearings may spill out and mix with the compressed air that not only can have an adverse effect on the airplane engine, but also will contaminate the air in the air-conditioned passenger cabin and is harmful to the health of passengers.    2. The conventional compressor attempts to solve the mechanical vibration problem of transmission shaft by using journal bearings. However, it requires an independent oil control system for controlling oil flow and pressure accurately that will increase the manufacturing cost of the compressor.    3. The specification of the roller bearing used in a conventional compressor restricts the rotation speed of the conventional compressor, which can not meet the direct driven speed, i.e. 28000˜33000 rpm, by gas turbine.